1. Field of the Invention
The present invention relates generally to honeycomb structures and the adhesives which are used to bond the layers of the honeycomb together. More particularly, the present invention involves the development of new node adhesives which offer a number of advantages over existing rubber or thermoplastic toughened thermoset adhesives.
2. Description of Related Art
Honeycomb structures are well known and widely used in many applications where a high strength and lightweight material is required. The combined features of lightweight and strength found in honeycomb structures makes them particularly well suited for use in aircraft and other applications where high strength and low weight are required. Honeycomb structures have been made from a wide variety of materials including metals, such as aluminum. Composite materials made from resin impregnated fibers and papers have also been widely used in honeycomb structures. Thermosetting resins have typically been used as the resin matrix of choice in cases where the honeycomb is used as part of a structural member where optimum honeycomb strength is required.
One common process for fabricating honeycomb structures involves bonding multiple sheets of material together along specially oriented node lines. The node lines are offset between different layers in such a way that a honeycomb structure is formed when the layers are expanded. This type of process is commonly referred to as the xe2x80x9cexpansionxe2x80x9d process. The expansion process is not suitable for fabricating honeycomb structures in some instances where certain thermoset matrix resins are used. For example, sheets made from certain materials which are too stiff or porous cannot be formed into honeycomb structures using the expansion process.
A fabrication process or method commonly referred to as the xe2x80x9ccorrugationxe2x80x9d process has been used to form high strength thermoset honeycomb structures in those situations where the expansion process cannot be used. The corrugation process involves initially shaping sheets of uncured thermoset or thermoplastic material into a corrugated configuration. The corrugated sheets are cured at high temperature to form stiff corrugated sheets which are then bonded together to form the honeycomb core. The honeycomb core is then optionally coated with a matrix or dip resin. The honeycomb core is generally cut into numerous flat panels which can be used xe2x80x9cas isxe2x80x9d or further processed in accordance with conventional honeycomb fabrication techniques. For example, the honeycomb core may be sandwiched between sheets of various materials to form extremely strong structural panels.
The adhesive which is used to bond the layers of the honeycomb together forms an important part of any honeycomb structure. Such adhesives are commonly referred to as xe2x80x9cnode adhesives.xe2x80x9d Node adhesives are critical because the ultimate strength of the honeycomb is often directly dependent upon the strength of the adhesive bond between the honeycomb layers. This is especially true in honeycombs which utilize high strength composite materials to form the honeycomb layers.
The importance of the node adhesive has led to a continual search for new and improved bonding materials. Thermoset resins that are toughened with rubber or thermoplastic particle have been found to be especially good node adhesives. These rubber or thermoplastic toughened thermoset adhesives are made by adding from 5 to 25 weight percent rubber or thermoplastic particles to a thermoset resin such as epoxy or cyanate ester resin. An organic solvent is also added to the adhesive mixture in order to maintain the viscosity of the node adhesive at a level which allows it to be processed and applied easily to the honeycomb nodes. N-methyl-2-pyrrolidone (NMP) and dimethylformamide (DMF) are the two principal solvents used in making toughened thermoset node adhesives. These solvents have been popular because they are able to dissolve a number of different thermoplastics and they can be removed from the adhesive by vaporization during the curing process.
Although the above-described node adhesives are well suited for their intended purpose, there continues to be a need for developing new and improved node adhesives which can be used advantageously to produce lightweight and structurally strong honeycomb structures.
In accordance with the present invention, improved thermoplastic toughened thermoset node adhesives have been developed which do not require a solvent. The solventless node adhesives provide a number of advantages over prior node adhesives which contain a solvent. It was discovered that evaporation of the solvent from the adhesive during high temperature curing produces small voids in the final adhesive bond which adversely affect the bond strength. Further, residual amounts of solvent usually remain in the node adhesive even when it is cured at temperatures up to 250xc2x0 C. and higher. The residual solvent detracts from overall bond strength and may present outgassing problems if the honeycomb is intended for use in a low pressure environment. Also, the solvent which evaporates during the heat forming process must be contained and disposed of in accordance with existing solvent vapor disposal procedures.
The present invention is based on the discovery that small thermoplastic particles may be incorporated into a thermosetting adhesive in amounts which are sufficient to provide a good node adhesive while at the same time not increasing the viscosity of the adhesive to levels which would require use of a solvent. It was found that small thermoplastic particles do not dissolve in thermosetting resins at room temperature. As a result, the thermosetting resin can be loaded with from 15 to 35 weight percent thermoplastic particles without increasing the viscosity of the mixture to a level which makes it unsuitable for use as a node adhesive. Upon curing at high temperature, the particles dissolve in the resin to provide a toughened resin which is especially well suited for use as a node adhesive.
The invention involves methods for making honeycombs wherein adjacent layers of the honeycomb are adhesively bonded together along a plurality of node lines. The solventless adhesive used to bond the adjacent layers of honeycomb together contains a thermoset resin, a curing agent for the thermoset resin and particles of thermoplastic resin which are suspended in the thermoset resin. The amount of thermoplastic particles suspended in the thermoset resin, as well as the temperature at which the adhesive is applied, is selected to provide a node adhesive which has a viscosity of between 10 to 180 poise at room temperature. Upon curing, the resulting honeycomb nodes have strong adhesive bonds which contain minimum voids and contains no residual solvent. The solventless node adhesive retains its strength even at elevated temperatures.
The above discussed and many other features and attendant advantages of the present invention will become better understood by reference to the following detailed description when taken in conjunction with the accompanying drawings.